1. Field of the Invention
The present invention relates to method for manufacturing alignment mark of semiconductor device, and in particular to an improved method for manufacturing alignment mark wherein a predetermined thickness of a device isolation film is etched prior to removing a pad nitride film during a STI (Shallow Trench Isolation) process to increase contrast for improving recognition capability of the alignment mark, thereby improving yield and reliability of semiconductor device.
2. Description of the Background Art
Generally, a semiconductor device comprises an active region where devices are formed and a device isolation region for defining the active region. In order to align masks accurately during subsequent processes, alignment marks are formed. One of the methods for forming alignment marks is by using STI trenches. That is, STI trenches filled with a device isolation film are formed and contrast generated due to a step difference between the device isolation film and the active region is compared to perform an alignment.
FIGS. 1a through 1c are cross-sectional diagram illustrating a conventional method for manufacturing alignment mark of semiconductor device. Only an alignment mark region is shown in FIGS. 1a through 1c. 
Referring to FIG. 1a, a pad oxide film (not shown) and a pad nitride film (not shown) are sequentially formed on a semiconductor substrate 10. The pad nitride film and the pad oxide film are etched via a photoetching process using a device isolation mask to form a pad nitride film pattern 14 and a pad oxide film pattern 12. The semiconductor substrate 10 is then etched using the pad nitride film pattern 14 as a mask to form an alignment mark trench 16.
Thereafter, a well oxide film 18 and a liner nitride film 20 are formed on an inner wall of the alignment mark trench 16. An oxide film for device isolation film (not shown) filling the alignment mark trench 16 is then formed on the entire surface. The oxide film for device isolation film is planarized until the pad nitride film pattern 14 is exposed to form a device isolation film 22.
Referring to FIG. 1b, the pad nitride film pattern 14 is removed.
Now referring to FIG. 1c, the device isolation film 22 is etched via a photoetching process using an alignment mark mask to form an alignment mark 24.
FIG. 2 is a photograph showing a plane view of various alignment marks. An alignment process may be performed by recognizing contrast generated due to a step difference between these alignment marks and adjacent layers. That is, in accordance with the conventional method for manufacturing alignment mark shown in FIGS. 1a through 1c, a predetermined thickness of the device isolation film is etched to increase the step difference between the device isolation film and the active region so that the contrast ratio is increased. However, in a CMP (Chemical Mechanical Polishing) process using HSS (High Selectivity Slurry), the thickness of the pad nitride film is relatively small in order to obtain better gap-filling characteristics. In this case, only small step difference is generated during the removal process of the pad nitride film pattern as shown in FIG. 3. Therefore, an accurate alignment in subsequent process using the contrast ratio is not possible due to the small step difference.